1. Field of the Invention
This invention is directed to a process for producing enriched gases from a vent gas stream which comprises hydrogen, low alkanes, aromatics and alkyl aromatics. More particularly, the present invention relates to a process for recovering ethane from a thermal hydrocracking vent gas, the ethane being suitable for use in an ethylene plant.
2. Description of the Prior Art
In thermal hydrocracking processes such an hydrodealkylation (HDA) which is used for producing benzene from an aromatic-rich feedstock or the modified unit (MBE) that produces benzene, ethane and methane from a feedstock of naphtha or gas oil, there is within the hydrocracking reactor a partial pressure of hydrogen that must be maintained to prevent coking and to sustain the reaction. For this reason, it is necessary to remove from the system the light hydrocarbons formed. This is accomplished either by venting the off-gas, which contains the light hydrocarbons, to fuel gas or, if the overall plant has a short supply of hydrogen, by purifying the off-gas and recycling the hydrogen-rich gas back to the thermal cracking reactor.
The recovery of hydrogen, hydrocarbons (as fuel gas), and aromatics by cryogenic means are known in the art. For example, U.S. Pat. No. 3,622,504 discloses a method for separating heavier hydrocarbons from natural gas by first condensing the hydrocarbons, and then flash-separating the mixture. U.S. Pat. No. 3,628,340 is directed to a process for separating condensable contaminants, such as methane, from a crude hydrogen stream. This process utilizes a series of multipass heat exchangers through which the gas flows for stepwise cooling, with interstage separation of condensates which are expanded and passed in a reverse flow path for autogenous refrigeration. U.S. Pat. No. 3,691,779 discloses a process for producing high purity hydrogen. The process comprises a low temperature refrigeration system operating below 120.degree. R., and an adsorption system operating on an adiabatic pressure-swing principle within the temperature range of 200.degree. to 140.degree. R.
It has been found that ethane can be readily recovered, purified and enriched to a high degree by flashing, heating, and re-condensing between two temperature points that can be readily controlled without external assistance.
Thus, a broad concept of the present cryogenic system is as follows:
(a) Providing a dry cryogenically acceptable feed gas having a pressure between 450 psia to 1000 psia; PA1 (b) Cooling the feed gas through a series of phase separators to recover alkyl aromatics, aromatics, lower alkanes and hydrogen; PA1 (c) Throttling or flashing the hydrocarbon contents to lower pressures to supply the Joule Thompson-type refrigeration; and further throttling the hydrocarbon contents to form a condensate liquid which is rich in ethane. PA1 (a) cooling the gaseous mixture to condense the aromatics; PA1 (b) separating the condensed aromatics from the gaseous mixture; PA1 (c) cooling the resulting gaseous mixture to condense the ethane; PA1 (d) separating the condensed ethane from the gaseous mixture of (c); PA1 (e) cooling the gaseous mixture of (d) to condense the methane; PA1 (f) separating the condensed methane from the gaseous mixture which is primarily composed of hydrogen; PA1 (g) recycling the condensed methane and gaseous mixture of (f) for use as coolants in steps (a), (c), and (e); PA1 (h) warming the ethane of (d) to produce a gas-liquid mixture by using the ethane as a coolant in (b); PA1 (i) flashing the gas-liquid mixture of (h) to separate the gas and liquid phases, the methane being in the gas phase and the ethane in the liquid phase; and PA1 (j) recycling the methane and ethane obtained in (i) for use as coolants in steps (a) and (c).